1. Technical Field
The present invention relates to a driving control method and system of a fuel cell system, and more particularly, to a driving control method of a fuel cell system capable of improving cold start performance by adjusting hydrogen pressure at an anode side.
2. Description of the Related Art
A fuel cell system used for a hydrogen fuel cell vehicle, which is a type of environment-friendly vehicles, is configured to include a fuel cell stack that generate electrical energy from an electrochemical reaction of reaction gases, a hydrogen supplying apparatus that supply hydrogen, which is fuel, to the fuel cell stack, an air supplying apparatus that supplies air including oxygen, which is an oxidizer necessary to perform the electrochemical reaction to the fuel cell stack, and a heat and water managing system that optimally adjusts a driving temperature of the fuel cell stack by discharging heat, which is a by-product of the electrochemical reaction of the fuel cell stack to the exterior and performing a water managing function.
A polymer film of the fuel cell stack should secure ion conductivity to increase performance of electrochemical reaction of hydrogen and water. As a degree of hydrolysis is increased, a reaction ratio of hydrogen and water is increased. Therefore, the hydrogen supplying apparatus has a hydrogen re-circulation system and the air supplying apparatus has a humidifier. However, water generated by a reaction with water supplied by humidification is frozen in a fuel cell when a temperature of the fuel cell decreases to 0° C. or less. When water remaining in the fuel cell is changed to an ice state, a volume thereof is expanded thus causing potential damage to a membrane electrode assembly and a gas diffusion layer having a pore structure. In addition, upon cold-starting, the generated water is frozen in an electrode of the fuel cell and is not discharged until it is thawed. The ice that is not discharged blocks a moving passage of reaction gas. To more stably drive the fuel cell vehicle after being cold-started, the ice is required to be thawed before the moving passage of the reaction gas in the fuel cell is fully blocked. Accordingly, an amount of water present in the fuel cell is required to be decreased before being cold-started.
FIG. 1 is an exemplary graph illustrating a change according to a temperature of a fuel cell stack and a time of an output of the fuel cell stack. Referring to FIG. 1, the temperature of the fuel cell stack is continuously increased as time passes. However, as shown in FIG. 1, as the temperature of the fuel cell stack is increased, a voltage of the fuel cell stack is increased, and during an ice blocking phenomenon, that is, a phenomenon in which the generated water is frozen to block the moving passage of the reaction gas, occurs, the voltage of the fuel cell stack is decreased. When the temperature of the fuel cell stack is increased to complete the thaw of the fuel cell stack, the voltage of the fuel cell stack is increased based on the increase in the temperature of the fuel cell stack.
To decrease the amount of water present in the fuel cell stack before being cold-started, the remaining water in the fuel cell during the driving is maintained to a predetermined amount or less or the water is removed through purge after being shut-down. By the above-mentioned process, the time in which the ice blocking phenomenon is observed upon being cold-started may be delayed, and a phenomenon in which a channel, which is the moving passage of the reaction gas, is blocked by the ice may be mitigated. To measure the amount of water present in the fuel cell, a method of measuring resistance in the fuel cell and a method of using experimental data obtained from a driving environment of the fuel cell stack may be used.
Meanwhile, when a driving temperature of the fuel cell is substantially low, saturated vapor pressure of an outlet portion of the fuel cell stack is substantially low. Therefore, an amount of discharged water is decreased, to increase an amount of remaining water. Therefore, a flooding phenomenon occurs in the fuel cell stack, thus increasing the amount of water to be removed.
FIG. 2 is an exemplary graph illustrating a change in an amount of water remaining in the fuel cell according to the temperature of the fuel cell stack. As illustrated in FIG. 2, as the temperature of the fuel cell stack is decreased, the amount of water remaining in the fuel cell stack is increased. Water remaining in a cathode and an anode prevents formation of a stack voltage upon being cold-started, thereby suppressing heating in the cell. Particularly, when the voltage of the fuel cell stack upon being cold-started decreases to below a minimum reference voltage based on the water remaining at the anode side, carbon in catalyst is changed to carbon dioxide in an anode electrode, such that a catalyst amount may be decreased.